For those who work on Mac, I have found a very powerfull 2D/3D program :Turbo3d . Visit the download page
Dont worry if it is a french page, the software perfectly works in English.
If you work on an Intel mac , download the Universal version. The other version if dedicated to G5 machines.
The program imports a lot of 3D formats ( OBJ included) and has an STL export for stereolithography. This is what we need , isnt'it ?

... the Lego Toys, Lego Technics, Toolo, Duplo... are all designed with Turbo 3D ...
... the greening of the White House have been done with Turbo3D...
... the Yerba Buana gardens and the Mosconi Center in SF have been designed with Turbo3D...
...Many parts of the CERN LHC have been designed with Turbo3D....
... and so many others...

I like ViaCAD, too. Maybe not as much features as in the more expensive programs, but very good and clean user interface and snap ideas. I've never used it before and I'm a beginner in 3D design, but I was able to learn it and to create this model in one day (but I know the Adobe Flash authoring tool and some things are familiar).